BGE: Adding an object linked from another file
I just reproduced a working example with two files, see the sceenshot below:



The left file is the library, containing an object "Suzi" which is part of the group "grpSuzi". I set up some simple rigid body physics here.

The right file is the "game", containing an empty named "spawn" which has the logic attached as seen in the screenshot. I added the "grpSuzi" on Layer two in this file. It made no difference whether I wrote "Suzi" or "grpSuzi" in the addObject() call. Once I pressed "P" in the game file (right) and the spacebar, the mesh appeared (twice). 

But once I misspelled the name like "Suz", I ended up with the error you described. Are you sure there is no typo?!